1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a method and apparatus for measuring the response time of the liquid crystal of which the optimal response time is automatically derived when the temperature of the liquid crystal is changed. Also, the present invention relates to a method and apparatus for driving a liquid crystal display device that might minimize the deterioration of picture quality which is generated when temperature of the liquid crystal display device is changed on the basis of the optimal response time derived by the method and apparatus of measuring the response time of the liquid crystal.
2. Description of the Related Art
In general, a liquid crystal display device controls light transmissivity of liquid crystal cells in accordance with video signals to display pictures. An active matrix type of liquid crystal display device, where a switching device is formed in each liquid crystal cell, has shown to be suitable for displaying motion pictures. The switching devices used in the active matrix type of liquid crystal display device is generally a thin film transistor (TFT).
The liquid crystal display device, as shown in the following formulas 1 and 2, has a disadvantage that its response time is slow due to its properties such as the unique viscosity and elasticity of liquid crystal.
                              τ          r                ∝                              γ            ⁢                                                  ⁢                          d              2                                                          Δ              ⁢                                                          ⁢              ɛ                        |                                          V                a                2                            -                              V                F                2                                      |                                              [                  Formula          ⁢                                          ⁢          1                ]            τr represents rising time when voltage is applied to liquid crystal, Va represents impressed voltage, VF represents Freederick Transition Voltage where liquid crystal molecules start to make tilt motion, and d represents the cell gap of liquid crystal cell, and γ represents the rotational viscosity of liquid crystal molecule.
                              τ          f                ∝                              γ            ⁢                                                  ⁢                          d              2                                K                                    [                  Formula          ⁢                                          ⁢          2                ]            τf represents the falling time during which liquid crystal is restored to its original position by elastic restoration after the voltage applied to the liquid crystal is turned off, and K represents the unique elastic modulus of liquid crystal.
The response time of liquid crystal of twisted nematic TN mode, which is a widely used liquid crystal mode in the liquid crystal display device, can be changed in accordance with the physical properties and cell gap of liquid crystal material, but generally its rising time is 20˜80 ms and its falling time is 20˜30 ms.
FIG. 1 is a diagram showing changes in brightness in accordance with data in a liquid crystal display device according to the related art. In FIG. 1, the response time of liquid crystal of twisted nematic TN mode is extended to the next frame before the voltage being charged the liquid cell with reaches a desired voltage, because the response time is longer than one frame period (NTSC: 16.67 ms), thus there appears the motion blurring phenomenon that screen gets blurred in motion pictures. In addition, a display brightness BL does not reach the desired brightness, so desired color and brightness are not able to be expressed, wherein the display brightness corresponds to the change of data VD from one level to another lever due to slow response time. As a result, the liquid crystal display device has motion blurring phenomenon appearing in motion picture and its picture quality going down due to the deterioration of contrast ratio.
In order to resolve the slow response time of such a liquid crystal display device, U.S. Pat. No. 5,495,265 and PCT international publication No. WO 99/05567 have introduced a scheme (hereinafter ‘high speed driving method’) that data are modulated in accordance with the existence or absence of the change of the data by use of a look-up table.
FIG. 2 is a diagram showing changes in brightness in a liquid crystal display device driven by a high speed driving method according to the related art and FIG. 3 is a diagram showing an example of eight-bit data using the high speed driving method according to the related art. In FIG. 2, the high speed driving method according to the related art modulates input data VD and applies the modulated data MVD to get desired brightness MBL. The high speed driving method has the value of |Va2-VF2| in formula 1 on the basis of the existence or absence of change of the data in order to get the desired brightness corresponding to the brightness value of the input data within one frame period. Accordingly, the liquid crystal display device using the high speed driving method compensates the slow response time of liquid crystal by modulating the data value to ease motion blurring phenomenon in motion pictures, thereby displaying pictures in the desired color and brightness.
In other words, the high speed driving method selects modulated data Mdata corresponding to input data in a look-up table and modulates them as in FIG. 3 if there is any change between the most significant bit MSB data of a previous frame Fn-1 and a current frame Fn when comparing their most significant bit MSB data. Such a high speed driving method modulates only upper few bits in order to reduce the burden of memory capacity when realizing hardware.
FIG. 4 is a block diagram of a high speed driving apparatus according to the related art. In FIG. 4, the high speed driving apparatus according to the related art includes a frame memory 43 connected to an upper bit bus line 42 and a look-up table 44 commonly connected to the output terminals of the frame memory 43 and the upper bit bus line 42. The frame memory 43 stores most significant bit MSB data for one frame period and supplies the stored data to the look-up table 44. Herein, the most significant bit MSB data is set to be upper four bits of an eight bit source data RGB Dataln.
The look-up table 44 compares the most significant bit MSB data of the current frame Fn inputted from the upper bit bus line 42 with the most significant bit MSB data of the previous frame Fn-1 inputted from the frame memory 43, as in table 1, and selects a modulated data Mdata corresponding to the comparison result. The modulated data Mdata is added to the least significant bit LSB data from a lower bit bus line 41 to be applied to the liquid crystal display device. Table 1 represents one example of the look-up table 44 where the most significant four bits 24, 25, 26, 27 of the previous frame Fn-1 are compared with the most significant four bits 24, 25, 26, 27 of the current frame Fn to select the modulated data Mdata corresponding to the comparison result.
In case that the most significant bit MSB data is set to be 4 bits, the look-up table 44 of high speed driving method is implemented as in Tables 1 and 2.
TABLE 10123456789101112131415 00234567910 1213141515151510134567810 1213141515151520024567810 1213141515151530013567810 111314151515154001346789111213141515155001235789111213141515156001234689101213141515157001234579101113141515158001234568101112141515159001234567 911121314151510 001234567 810121314151511 001234567 8 9111314151512 001234567 8 9101214151513 001233456 7 8101113151514 001233456 7 8 91112141515 000123345 6 7 8 9111315
TABLE 20163248648096112128144160176192208224240 0032 48648096112144160192208224240240240240 16016 48648096112128160192208224240240240240 320032648096112128160192208224240240240240 480016488096112128160176208224240240240240 640016486496112128144176192208224240240240 800016324880112128144176192208224240240240 960016324864 961281441601922082242402402401120016324864 801121441601762082242402402401280016324864 80 961281601761922242402402401440016324864 80 961121441761922082242402401600016324864 80 961121281601922082242402401760016324864 80 961121281441762082242402401920016324864 80 961121281441601922242402402080016324848 64 80 961121281601762082402402240016324848 64 80 9611212814417619222424024000 0163248 48 64 80 96112128144176208240
In Tables 1 and 2, the leftmost column represents the data voltage VDn-1 of the previous frame Fn-1 and the uppermost row represents the data voltage VDn of the current frame Fn. Table 1 represents the information of a look-up table where most significant four bits 24, 25, 26, 27 are expressed in a decimal numeral. Table 2 represents the information of a look-up table in case that the weight of the most significant four bits 24, 25, 26, 27 in an eight bit data is applied to the data of Table 1.
However, the high speed driving method has a problem that its effect comes out differently in accordance with the category temperature of liquid crystal display device. Such a problem has been confirmed by an experimental result conducted using a trial product of 30″ liquid crystal display module with a resolution of 1280×768 which is made by the applicant of this invention and is on trial sale.
Table 3 represents the response time (ms) of rising time and falling time for each of gray scales 0(G0), 63(G63), 127(G127), 191(G191), 255(G255) in case that the above trial product is driven at 0° C. in a normal driving method like FIG. 1.
TABLE 3Rising timeFalling timeG255G191G127G63G0G25526.729.331.031.1G19150.359.661.563.5G12745.951.261.667.9G6337.140.846.164.4G027.025.524.326.0
Table 4 represents the response time (ms) of rising time and falling time for each of gray scales 0(G0), 63(G63), 127(G127), 191(G191), 255(G255) in case that the above trial product is driven at 0° C. in the high speed driving method.
TABLE 4Rising timeFalling timeG255G191G127G63G0G25527.629.231.431.1G19145.949.254.557.8G12743.444.859.865.0G6336.537.042.255.8G024.624.223.624.7
As shown in Tables 3 and 4, there is almost no difference in the rising time of liquid crystal cells between when the above trial product is driven at the using environment of 0° C. in the high speed driving method and when the above trial product is driven at the using environment of 0° C. in the normal driving method as in FIG. 1. In other words, there is difficulty in making the response time fast at a low temperature environment even if the liquid crystal display device is driven in the high speed driving method.
Table 5 represents the response time (ms) of rising time and falling time for each of gray scales 0(G0), 63(G63), 127(G127), 191(G191), 255(G255) in case that the above trial product is driven at 25° C. in the high speed driving method.
TABLE 5Rising timeFalling timeG255G191G127G63G0G25510.010.911.412.1G19111.011.811.611.4G12711.711.611.411.3G6311.712.011.511.5G09.168.48.17.6
As shown in Tables 4 and 5, even when making the response time of liquid crystal fast by driving the liquid crystal display device in the high speed driving method, the response time of liquid crystal gets remarkably slow to deteriorate its picture quality if the using environment of the liquid crystal display device is low (0° C.) in temperature. As a result, the conventional liquid crystal display device has its picture quality changed because the response time of liquid crystal is changed if its category temperature is changed even though it is driven in the normal driving method as in FIG. 1 or in the high speed driving method.